This invention relates generally to the field of medical devices, and more particularly to a method and system for discriminating and classifying supraventricular tachycardia and ventricular tachycardia events.
Recent prospective clinical trials have shown that cardioverter-defibrillators, such as implantable cardioverter-defibrillators (ICDs), reduce sudden arrhythmic death and favorably impact overall mortality in patients at risk for spontaneous ventricular tachyarrhythmia. Cardioverter-defibrillator systems are designed to provide therapy when rapid ventricular activation rates are sensed. However, rapid ventricular rhythms can occur in the presence of a supraventricular tachycardia (SVT). When therapy is applied in response to SVT (in absence of a ventricular tachycardia, VT, or ventricular fibrillation, VF), the therapy is classified as clinically xe2x80x9cinappropriatexe2x80x9d, even though the cardioverter-defibrillator responded appropriately to an elevated ventricular rate.
Cardioverter-defibrillators may deliver inappropriate ventricular therapy to patients afflicted with non-malignant SVTs. These inappropriate therapies may be delivered due to the device""s inability to reliably discriminate SVT from malignant VT.
For the reasons stated above, and for other reasons stated below which will become apparent to those skilled in the art upon reading and understanding the present specification, there is a need in the art for a system and a method of reliably and accurately discriminating between the occurrence of a SVT and a VT event during a detected tachyarrhythmia event which can reduce the frequency of inappropriate therapies delivered to cardioverter-defibrillator patients. Such a system may also be suitable for use with patients having implantable cardioverter-defibrillators.
The present system provides a means for discriminating, or classifying supraventricular tachycardias (SVT) from malignant ventricular tachycardias (VT). The present disclosure teaches a number of embodiments useful for, among other things, classifying a tachycardia or fast arrhythmia as either a SVT or a VT event. In one embodiment, the system utilizes a series of discrimination stages employing a plurality of methods for distinguishing and classifying VT and SVT. In one embodiment, stages are arranged so that the computationally more efficient stages are used initially in assessing and classifying the tachycardia event. In one embodiment, this multiple stage system allows for a more accurate assessment of the patient""s condition before treatment is delivered. Furthermore, this multiple stage system allows for earlier (i.e., faster) treatment of certain VT events, which, in the case of an implantable device, results in a more efficient use of the ICD""s battery.
In one embodiment, there is provided a system for classifying VT from SVT during a tachycardia event. Cardiac signals representative of electrical cardiac activity are sensed and analyzed of the occurrence of a tachycardia event. When a tachycardia event is detected, a plurality of features along the sensed cardiac signals are analyzed in two or more discrimination stages. In one embodiment, a discrimination stage is used to distinguish and classify the tachycardia event as either being a VT event or a candidate SVT event.
In one embodiment, the first discrimination stage analyzes the width of repeatably identifiable features on the sensed cardiac signals and compares them to a template value to classify the tachycardia event. In one embodiment, the first discrimination stage acts to measure a width of sensed R-waves using the plurality of features from cardiac signals sensed during a tachycardia event. The width of each of the sensed R-waves is then compared to a template R-wave width. In one embodiment, the template R-wave width is determined from cardiac signals sensed during the patient""s normal sinus rhythm.
If the comparison of the sensed R-waves to the template R-wave width reveals that the width of a sensed R-wave is greater than or equal to a predetermined value of the template R-wave width, the cardiac signal is classified as a ventricular tachycardia complex. As the cardiac signals are classified, the number of ventricular tachycardia complexes are recorded, and when the number of ventricular tachycardia complexes reach a predetermined threshold a ventricular tachycardia is declared. Once a ventricular tachycardia is declared, therapy for treating the ventricular tachycardia is delivered to the patient.
If in the first discrimination stage a ventricular tachycardia is not declared, a second discrimination stage is then used to assess and classify the tachycardia event. In one embodiment, the second discrimination stage includes the acts of determining values for each of the plurality of features of the cardiac signals sensed during a tachycardia event. In one embodiment, the plurality of features includes the value of maximum and minimum deflection points along the sensed cardiac signals.
The values for the plurality of features for each of the cardiac signals sensed during the tachycardia event are then used in determining a similarity value and a dissimilarity value. In one embodiment, the similarity value and the dissimilarity value indicate the similarity of the sensed cardiac signal to cardiac signals sensed during normal sinus rhythm. As such, the similarity value and the dissimilarity value for the sensed cardiac complexes are assessed relative to a plurality of features on normal sinus rhythm signals.
The similarity value and the dissimilarity value are then used to determine if each of the cardiac signals is a ventricular tachycardia complex. In one embodiment, this is accomplished by plotting the similarity value and the dissimilarity value on a discrimination plane. Based on where the cardiac signal is plotted on discrimination plane, the cardiac signal is either classified as a ventricular tachycardia complex or a candidate supraventricular tachycardia complex. In one embodiment, the candidate supraventricular tachycardia complex is also known as a non-ventricular tachycardia complex.
As the cardiac signals are classified, the number of ventricular tachycardia complexes are recorded, and when the number of ventricular tachycardia complexes reach a predetermined threshold a ventricular tachycardia is declared. In one embodiment, once a ventricular tachycardia is declared, therapy for treating the ventricular tachycardia is delivered to the patient.
In one embodiment, if in the first discrimination stage the tachycardia event is classified as a candidate supraventricular tachycardia (or a non-ventricular tachycardia) and the second discrimination stage also classifies the tachycardia event as a candidate supraventricular tachycardia, then the tachycardia event is declared a supraventricular tachycardia.
These and other features and advantages of the invention will become apparent from the following description of the embodiments of the invention.